The present invention relates to nucleic acid amplification reactions and in particular relates to amplification reactions that employ a pair of primer sequences to generate copies of a target sequence.
Nucleic acid amplification reactions are well known and are employed to increase the concentration of a target nucleic acid in a test sample. The xe2x80x9ctarget nucleic acidxe2x80x9d typically is present in a sample in low concentrations and therefore cannot easily be detected without amplifying it to increase the concentration of the target sequence in the sample. The polymerase chain reaction (PCR) is one nucleic acid amplification reaction commonly employed for purposes of amplifying a target nucleic acid sequence.
According to the principles of PCR, xe2x80x9cprimer sequencesxe2x80x9d are used to prime synthesis of copies of the target sequence. Specifically, under appropriate conditions, primer sequences hybridize to opposite strands of a double stranded nucleic acid sequence such that the primers flank the target sequence. Once hybridized, the primers are extended using enzymes such as, for example, DNA polymerase which extend the primer sequences to thereby generate copies of the target sequence. Additional copies of the target sequence are generated by cycling the above steps of (i) hybridizing and extending the primer sequences and (ii) dissociating the extended primer sequences (or copies of the target sequence) so that additional primers can hybridize to the original target, as well as copies of the target sequence. Hence, multiple copies of the target sequence are generated.
Once amplified, copies of the target sequence can be detected to determine if the target sequence originally was present in the test sample. Of course, if the target sequence was not present, amplification should not occur and the target sequence should not be detected. In any event, amplified target sequences are typically detected using labels. Labels are moieties that have a detectable property and can be incorporated into the copies of the target sequence. Labels typically are incorporated into the amplified target sequences by attaching the labels to primer sequences that are then incorporated into the amplification product as specified above. Alternatively, for example, extension products can be labeled by incorporating labeled nucleotides into such products during primer extension. The presence of the target sequence in the test sample can then be determined by detecting the labeled amplification product.
Amplified target sequences also can be detected using labeled probes that hybridize to a strand or both strands of an amplified target sequence. However, it is sometimes desirable to employ a probe that hybridizes to only one strand of a double stranded amplification product. The effect of such a detection scheme, at least as it applies to a double stranded target sequence, is that a single strand of amplified target sequence is detected to determine the presence of the target sequence in the test sample. However, detecting a single strand of an amplification product can be inefficient insofar as the signal plateaus and sometimes drops (or hooks) as the number of target sequences originally present in the test sample increases. Alleviating the xe2x80x9chookingxe2x80x9d or xe2x80x9cplateauingxe2x80x9d phenomenon and providing a linear signal over a broader range of target sequence concentrations would be beneficial, especially for amplification based assays designed to quantify the amount of a target sequence in a test sample.
It would be expected that substantially increasing the concentration of one primer over the other would alleviate this problem by generating more of the sequence that is detected. Indeed, U.S. Pat. No. 5,066,584 describes a method for preferentially generating one strand of a double stranded target sequence by vastly increasing the concentration of one primer. However, this requires excess reagents and therefore excess costs associated with preferentially producing one of two single strands. Additionally, substantially increasing primer concentrations may increase the chances of non-specific priming and therefore amplification of non-target sequences. Moreover, many times, competing non-specific reactions will interfere with the efficient amplification of the sequence of interest. Therefore, it may be expected that substantially increasing the concentration of one primer over the other primer may present problems in amplification assays designed to be of high sensitivity (i.e. designed to detect low numbers of a sequence of interest).
The present invention provides a method of detecting a target sequence in a test sample. The method comprises the steps of: (a) forming a reaction mixture comprising a test sample, amplification reagents, a first primer, and a second primer such that the concentration of the first primer in the reaction mixture is 15% to 250% percent greater than the concentration of the second primer; (b) amplifying the target sequence to generate copies of the target sequence comprising an amplification product from the first and second primers; (c) hybridizing a probe to the amplification product from the first primer to form a hybrid complex; and (d) detecting the hybrid complex as an indication of the presence of the target sequence in the test sample. Preferably, the hybrid complex is detected using labels that can either be directly detectable or indirectly detectable.
Also provided is an improved method for amplifying and detecting a target nucleic acid sequence in a test sample comprising the steps of: (a) forming an amplification mixture comprising a test sample, a first and a second primer sequence, and amplification reagents, (b) amplifying the target sequence to generate copies of the target sequence comprising an amplification product from the first and second primers; and (c) detecting the copies of the target sequence as an indication of the presence of the nucleic sequence in the test sample; wherein the improvement comprises providing the first primer sequence in 15% to 250% excess over the second primer and wherein a probe is hybridized to the amplification product from the first primer to form a hybrid complex and the hybrid complex is detected as an indication of the presence of the nucleic acid sequence in the test sample.
Kits for performing the methods of the invention are also provided.
Under appropriate conditions, a primer pair will generate copies of a target sequence in the form of a double stranded amplification product. Unfortunately, however, when a single strand of a double stranded amplification product is detected with a probe, the resulting signal can plateau, or even hook, as the concentration of the original target sequence increases. To a certain extent, this phenomenon is counterintuitive since increasing the concentration of the original target sequence should yield a greater concentration of end product, and therefore, a greater signal should be detected. However, as mentioned above, the resulting signal can plateau. While not wishing to be bound by theory, the hooking effect may be attributable to the presence of higher concentrations of longer product strands driving product strand re-annealing to the exclusion of probe/target strand annealing. Applicants have surprisingly and unexpectedly discovered that the plateauing or hooking phenomenon could be alleviated by increasing the concentration of one primer so that it is slightly higher than the concentration of the other primer.
The method provided herein can be applied to any amplification reaction where a pair of primer sequences is employed to generate double stranded amplification products and only one strand of the double stranded products is detected. The method comprises a step where an amplification mixture is formed. The amplification mixture generally will comprise (i) a test sample, (ii) amplification reagents and (iii) a first and second primer (collectively referred to as a xe2x80x9cprimer pairxe2x80x9d). As used herein, the term xe2x80x9ctest samplexe2x80x9d means anything suspected of containing a target sequence. The test sample is or can be derived from any biological source, such as for example, blood, ocular lens fluid, cerebral spinal fluid, milk, ascites fluid, synovial fluid, peritoneal fluid, amniotic fluid, tissue, fermentation broths, cell cultures and the like. The test sample can be used directly as obtained from the source or following a pre-treatment to modify the character of the sample. Thus, the test sample can be pretreated prior to use by, for example, preparing plasma from blood, disrupting cells or viral particles, preparing liquids from solid materials, diluting viscous fluids, filtering liquids, distilling liquids, concentrating liquids, inactivating interfering components, adding reagents, purifying nucleic acids, and the like. The xe2x80x9ctarget sequencexe2x80x9d that may be present in the test sample is a nucleic acid sequence that is amplified, detected, or amplified and detected. Additionally, while the term target sequence is sometimes referred to as single stranded, those skilled in the art will recognize that the target sequence may actually be double stranded.
The phrase xe2x80x9camplification reaction reagentsxe2x80x9d as used herein means reagents which are well known for their use in nucleic acid amplification reactions and may include but are not limited to: an enzyme or enzymes separately or individually having DNA polymerase and/or reverse transcriptase activity; enzyme cofactors such as magnesium or manganese; salts; nicotinamide adenine dinucleotide (NAD); deoxynucleoside triphosphates (dNTPs) such as, for example, deoxyadenosine triphosphate, deoxyguanosine triphosphate, deoxycytodine triphosphate and thymidine triphosphate; and an appropriate buffer.
The first and second primers typically are nucleic acid sequences, usually DNA or RNA. The length of the primers is not critical but primer sequences are usually about 10 to about 100 nucleotides long, preferably from about 15-35 nucleotides long, and have a defined base sequence suitable for hybridizing to the desired target sequence. Primer pairs usually are selected such that they flank the target sequence as is well known in the art. Additionally, the first primer is added to the amplification mixture such that its concentration is between 15% to 250% greater, and preferably 20% to 150% greater, than the concentration of the second primer. Of course, the first primer can be employed at concentrations of 400%, 500% and up to and more than 1000% greater than the concentration of the second primer, but as the concentration of one primer over the other increases, reagent costs and/or non-specific priming can become a limiting factor. In any event, upon hybridization of a primer to a target sequence, the primer is extended to generate a complement of the sequence to which the primer is hybridized.
Primer sequences can be from natural or synthetic sources and can routinely be synthesized using a variety of techniques currently available. For example, primers can be synthesized using conventional nucleotide phosphoramidite chemistry and instruments available from Perkin Elmer/Applied Biosystems, Div., (Foster City, Calif.) or Perceptive Biosystems, Inc., (Framingham, Mass.). If desired, a primer can be labeled using methodologies well known in the art such as described in U.S. patent application Ser. Nos. 5,464,746; 5,424,414; and 4,948,882 all of which are herein incorporated by reference.
After the amplification mixture is formed, the target nucleic acid is amplified by subjecting reaction mixture to xe2x80x9camplification conditionsxe2x80x9d which are conditions that promote amplification of the target sequence. Amplification conditions are well known to those skilled in the art and generally comprise conditions that promote dissociation of a double stranded target sequences, annealing of the primer sequences to the single strands of the target sequence, and extension of the primer sequences to thereby form copies of the target sequences. The copies of the target sequence are then dissociated from the target and additional primer sequences are annealed to both the original target sequence and copies of the target sequence to thereby start a new round of amplification of the target sequence. Such amplification conditions are well known and have been described in U.S. Pat. Nos. 4,683,202 and 4,683,195 both of which are herein incorporated by reference. Thermal cycling is a preferred and well known method for producing amplification conditions. The number of times an amplification mixture is cycled is a matter of choice for one skilled in the art and typically, a reaction mixture is cycled between 2 and 100 times and more typically between 20 and 40 times.
After cycling, multiple copies of the target sequence may be present. The sequence(s) generated by the first primer is the sequence that is detected to indicate the presence of the target sequence in the test sample and this sequence synthesized by the first primer is variously referred to herein as the xe2x80x9cprimary sequencexe2x80x9d. Any method for detecting a single strand of a double stranded amplification product can be employed according to the present invention. For example, sequencing, gel electrophoresis, gel shift assays, solution hybridization assays, xe2x80x9cTaqManxe2x80x9d like assays, and similar formats can be employed to detect the primary sequence.
According to a preferred detection embodiment, a hybridization probe is employed to detect the primary sequence, particularly when the probe is at relatively low concentrations. Probe sequences hybridize to the primary sequence to form a hybrid complex. Preferably, probes hybridize to the primary sequence in a region that is internal with respect to the primers. Formation of a hybrid complex between the primary sequence and probe can be accomplished by placing any double stranded target sequences under dissociation conditions followed by placing any resultant single stranded sequences under hybridization conditions in the presence of a probe. The phrase xe2x80x9cdissociation conditionsxe2x80x9d is defined generally as conditions which promote dissociation of double stranded nucleic acid to the single stranded form. These conditions can include high temperature and/or low ionic strength. The phrase xe2x80x9chybridization conditionsxe2x80x9d is defined generally as conditions which promote nucleation and annealing of complementary nucleic acid sequences. It is well known in the art that such annealing and hybridization is dependent in a rather predictable manner on several parameters, including temperature, ionic strength, sequence length and G:C content of the sequences. For any given set of sequences, melt temperature, or Tm, can be estimated by any of several known methods. Typically hybridization conditions include temperatures which are slightly below the melt temperature of a given set of nucleic acid sequences. Ionic strength or xe2x80x9csaltxe2x80x9d concentration also impacts the melt temperature, since small cations tend to stabilize the formation of duplexes by shielding the negative charge on the phosphodiester backbone. Typical salt concentrations depend on the nature and valence of the cation but are readily understood by those skilled in the art. Similarly, high G:C content and increased sequence lengths are also known to stabilize duplex formation because G:C pairings involve 3 hydrogen bonds where A:T pairs have just two, and because longer sequences have more hydrogen bonds holding the strands together. Thus, a high G:C content and longer sequence length impact what xe2x80x9chybridization conditionsxe2x80x9d will encompass. Based upon the above, determining the proper xe2x80x9chybridization conditionsxe2x80x9d for a particular set of nucleic acid sequences is well within the ordinary skill in the art. U.S. patent application Ser. No. 081514,704, filed Aug. 14, 1995, which is herein incorporated by reference, exemplifies a method of detecting amplified target sequences with a probe.
Probes are also nucleic acid sequences or nucleic acid analog sequences such as, for example, DNA, RNA, peptide nucleic acids, morpholino nucleic acids, that can be synthesized and labeled in the same manner that primer sequences are synthesized and labeled, as specified above. Selection of labels employed on a labeled primer or probe is a matter of choice for those skilled in the art and the term xe2x80x9clabelxe2x80x9d as used herein refers to a molecule or moiety having a property or characteristic which is capable of detection. A label can be directly detectable, as with, for example, radioisotopes, fluorophores, chemiluminophores, enzymes, colloidal particles, fluorescent microparticles, FRET pairs, and the like. Alternatively, a label may be indirectly detectable, as with, for example, specific binding members. It will be understood that directly detectable labels may require additional components such as, for example, substrates, triggering reagents, light, and the like to enable detection of the label. When indirect labels are used for detection, they are typically used in combination with a conjugate as will be discussed further below.
Probes can be employed in a variety of ways, known in the art, to detect the primary sequence. For example, the probe, primer or probe and primer can be labeled and/or immobilized to solid support materials to detect the presence of the primary sequence. Capture reagents also can be employed to aid in detecting a primary sequence. A xe2x80x9ccapture reagentxe2x80x9d as used herein means a specific binding member attached to a solid support material. xe2x80x9cSpecific binding memberxe2x80x9d as used herein, means a member of a specific binding pair, i.e. two different molecules where one of the molecules through, for example, chemical or physical means specifically binds to the other molecule. In addition to antigen and antibody specific binding pairs, other specific binding pairs include, but are not intended to be limited to avidin and biotin; complementary nucleotide sequences; haptens and antibodies specific for haptens such as carbazole and adamantane described in U.S. Pat. No. 5,424,414 and U.S. Pat. No. 5,464,746, respectively (the disclosures of these patents are incorporated herein by reference); and the like. A xe2x80x9csolid support materialxe2x80x9d, refers to any material which is insoluble, or can be made insoluble by a subsequent reaction. Solid support materials thus can be latex, plastic, derivatized plastic, magnetic or non-magnetic magnetic metal, glass, silicon or the like. A vast array of solid support material configurations are also well known and include, but are not intended to be limited to, surfaces of test tubes, microtiter wells, sheets, beads, microparticles, chips and other configurations well known to those skilled in the art.
According to one embodiment for detecting a primary sequence using a probe, the probe can be immobilized to a solid support material to form a capture reagent. The primary sequence can be contacted with the so-formed capture reagent under hybridization conditions to form a hybrid complex and thereby capture the primary sequence and, if desired, separate it from other amplification reactants and products. A signal from a label attached to the primary sequence can then be detected as an indication of the presence of the primary sequence on the capture reagent and therefore in the test sample.
As a further alternative, the first primer and probe can be labeled and the primary sequence can be separated and detected using such labels. For example, both labels of such a configuration can be specific binding members. Hence upon formation of a hybrid complex, the complex will be bi-labeled. One label can bind to a specific binding member on a capture reagent that permits separation of the hybrid complex and the other label can be used to bind a conjugate which can be employed to detect the presence of the hybrid complex on the capture reagent. The term xe2x80x9cconjugatexe2x80x9d as used herein means a specific binding member that has been attached or coupled to a directly detectable label. Coupling chemistries for synthesizing a conjugate are well known in the art and can include, for example, any chemical means and/or physical means that does not destroy the specific binding property of the specific binding member or the detectable property of the label.